1. Field of the Invention
The present invention relates to a secondary battery protection semiconductor device for protecting a secondary battery such as a lithium-ion battery from becoming abnormal, which secondary battery is built in a battery pack used in portable electronic devices, and also to the battery pack using the secondary battery, and to various electronic devices.
2. Description of the Related Art
The recent years have witnessed the increasing pervasiveness of various electronic devices such as mobile phones, notebook computers, and PDAs (Personal Digital Assistants). In these electronic devices, battery packs are widely used as the power source as they are easy to handle.
In a battery pack, one or more secondary batteries are stored in a single package. High capacity batteries are used as the secondary batteries, such as lithium-ion batteries, lithium-polymer batteries, or nickel-hydride batteries. An extremely large amount of energy is held inside a high capacity battery. Therefore, when this battery enters an overcharge, overdischarge, or overcurrent state, the battery generates heat, or even sometimes catches fire.
Thus, the battery pack accommodates a secondary battery protection semiconductor device for protecting the secondary battery from entering overcharge, overdischarge, charge overcurrent, discharge overcurrent, short-circuit current, and anomalously heated (overheat) states. When the above protection is necessary, the connection is cut between the secondary battery and the battery charger or the load device, in order to prevent the secondary battery from generating heat or catching fire, and also to prevent the secondary battery from degrading.
For the purpose of detecting overcharge, overdischarge, charge overcurrent, discharge overcurrent, short-circuit current, and overheat states, the secondary battery protection semiconductor device includes detection circuits dedicated to each of these states. When any of these detection circuits detects an abnormal level which requires the protection operation, the corresponding detection circuit outputs a detection signal, and turns off the switch unit (charge control transistor, discharge control transistor) provided between the secondary battery and the battery charger or the load device, in order to cut the connection therebetween.
A conventional method for overheat protection is disclosed in Japanese Laid-Open Patent Application No. 2007-124775 (patent document 1). FIG. 6 illustrates a battery pack 103 with a built-in secondary battery protection device and a secondary battery 102 shown in FIG. 1 of patent document 1.
As shown in FIG. 6, the secondary battery protection device (dashed line in FIG. 6) of patent document 1 includes a control circuit 111, an overcharge detection circuit 112 for detecting an overcharge state, an overdischarge detection circuit 113 for detecting an overdischarge state, an overcurrent detection circuit 114 for detecting an overcurrent state, an overheat detection circuit 115 for detecting an overheat state, a temperature sensor 116 for measuring the temperature, a discharge control transistor Q1 for controlling discharging operations, a charge control transistor Q2 for controlling charging operations, and parasitic diodes D1 and D2 for the discharge control transistor Q1 and the charge control transistor Q2, respectively. Furthermore, the battery pack 103 includes the above-described secondary battery protection circuit and the secondary battery 102.
A brief description is given of operations of the secondary battery protection device shown in FIG. 6. Output signals from the overcharge detection circuit 112, the overdischarge detection circuit 113, and the overcurrent detection circuit 114 are input to the control circuit 111. When the control circuit 111 detects that some abnormality has occurred based on the output signals from the various detection circuits, the control circuit 111 turns on/off the discharge control transistor Q1 and the charge control transistor Q2 in accordance with the output from the detection circuit that has detected the abnormality, in order to protect the secondary battery 102.
Furthermore, when the overheat detection circuit 115 has detected an overheat state upon referring to the temperature detected by the temperature sensor 116, the control circuit 111 turns on/off the discharge control transistor Q1 or the charge control transistor Q2 to protect the secondary battery 102, the discharge control transistor Q1, and the charge control transistor Q2.
The target of detecting overheat may be the main unit of the secondary battery 102 or the discharge control transistor Q1 and the charge control transistor Q2, or all of these elements. A short-circuit detection circuit for detecting a short-circuit current may be provided in addition to the overcharge detection circuit, the overdischarge detection circuit, and the overcurrent detection circuit. Such a configuration is disclosed in, for example, Japanese Laid-Open Patent Application No. 2006-121900 (patent document 2).    Patent document 1: Japanese Laid-Open Patent Application No. 2007-124775    Patent document 2: Japanese Laid-Open Patent Application No. 2006-121900
In the conventional secondary battery protection circuits described above, the detection levels of the overcharge detection circuit 112, the overdischarge detection circuit 113, the overcurrent detection circuit 114, and the overheat detection circuit 115 are set separately from one another, and there is no association between these detection items. The control circuit 111 separately evaluates the different output results from the various detection circuits, and switches on/off the discharge control transistor Q1 and the charge control transistor Q2 based on the evaluation.
However, some detection items may affect other detection items. Particularly, when the temperature of the secondary battery 102 is high, the voltage detection level for detecting overcharge is preferably set to a low value in order to mitigate damage inflicted on the secondary battery 102.
When the temperatures of the discharge control transistor Q1 and the charge control transistor Q2 are high, the overcurrent detection levels while charging and discharging are preferably set to low values. Conversely, when the temperatures of the discharge control transistor Q1 and the charge control transistor Q2 are low, the overcurrent detection levels while charging and discharging are preferably increased to high values.